With increases in the recording capacity of magnetic disk devices, the distance between the magnetic disk serving as a recording medium and the head for recording information or playback becomes almost nil close to contact therebetween. Disks are rotated also at increased speeds, so that the contact between the disk and the head tends to produce an ever increasing impact.
To reduce the impact of contact, efforts have been made to provide carbon protective films which are appropriate in surface roughness and strength and lubricant films having an appropriate thickness and to ensure a proper composition ratio between the bonded layer and the mobile layer. Especially the lubricant film which is positioned on the outermost surface of the disk needs to satisfy requirements which include, besides the reduction of impact of contact, prevention of the contamination of the disk surface and prevention of transfer of the film to the head. In addition to the thickness of the lubricant film and the composition ratio between the bonded layer and the mobile layer, therefore, also important is the design of the chemical structure of the lubricant.
The lubricants generally in use are perfluoropolyethers having functional groups. Examples of functional groups are hydroxyl, amino and cyclophosphazene group. The monomer units providing the main chain of perfluoropolyether include, for example, CF2O, CF2CF2O, CF(CF3)CF2O and CF2CF2CF2O. Homopolymers or copolymers of such units provide main chains of perfluoropolyethers.
Fomblin ZTETRAOL of the formula (2) is available as a lubricant comprising a compound having a main-chain structure of CF2O. This lubricant exhibits good properties to adhere to the disk due to the presence of hydroxyl at opposite terminals of the molecule, but the CF2O portion providing the main chain is low in resistance to Lewis acids, reacts with Al2O3 in the component of the head and becomes cleaved in its main chain (see, for example, Nonpatent Literature 1). When this cleavage progresses, the compound becomes lower in molecular weight, eventually vaporizing off from the magnetic disk surface and failing to retain the form of a lubricant film in systems involving contact or sliding movement between the head and disk
wherein m and n are each a real number of 10 to 40.
On the other, lubricants of the formula (3) or formula (4) are available as lubricants comprising a compound wherein the main-chain structure has CF2CF2CF2O only. These lubricants are less likely to decompose even in the presence of Lewis acids, affording magnetic disks of improved durability when applied as a lubricant layer (see, for example, Patent Literatures 1 and 2).HO—CH2CF2CF2O(CF2CF2CF2O)pCF2CF2CH2—OH  (3)wherein p is an integer, and the lubricants are 800 to 5000 in number average molecular weight
wherein q is an integer, r is an integer of 1 to 5, and the lubricants are 500 to 4000 in number average molecular weight.
With recent trends toward low flying-heads and high-speed disk rotation, it is required that the lubricant to be positioned on the outermost disk surface be prevented from spattering and decomposition that would lead to the disappearance of the lubricant layer and exhibit mobility for diminishing the frictional force that will result from the contact of the head with the disk so as to give improved durability to the disk.
However, when used singly, the compound (3) or compound (4) of the above-mentioned Patent Literatures 1 and 2 was unable to give improved durability to disks as will be apparent from Experimental Example 1 given later.    [Nonpatent Literature 1] Macromolecules, 1992, Vol. 25, p. 6791-6799    [Patent Literature 1] JP 1996-63739 A    [Patent Literature 2] JP 1998-149527 A
An object of the present invention is to provide a lubricant which is excellent in both adhesion and mobility and outstanding in durability when used for high-speed rotation disks and low-flying heads, and to provide magnetic disks having the lubricant applied thereto.